1. Field of the Invention
The present invention relates to sputtering targets which are useful for depositing ferroelectric films and which are free of crack generation even when used to deposit a thin film for a DRAM or the like containing a ferroelectric oxide having the perovskite structure.
2. Discussion of the Background
As is well-known, a sputtering target composed of a sintered material including a compound oxide of Ba and Ti (hereinafter referred to as BaTi compound oxide), a compound oxide of Sr and Ti (hereinafter referred to as SrTi compound oxide), or a compound oxide of Ba, Sr and Ti (hereinafter referred to as BaSrTi compound oxide) is generally used to form a memory-retaining capacitor thin film for a dynamic random access memory (DRAM). Further, it is also well-known that any ferroelectric film which is formed of a BaTi, SrTi, or BaSrTi compound oxide by sputtering such a target has the perovskite structure, and that particularly a sputtering target composed of a BaSrTi compound oxide is most widely used to form a memory-retaining capacitor thin film for a DRAM.
A sputtering target composed of a sintered material including such a BaTi, SrTi, or BaSrTi compound oxide is manufactured as follows. Initially, a BaCO.sub.3 powder, a SrCO.sub.3 powder, and/or a TiO.sub.2 powder are prepared as raw-material powders, and then blended in a predetermined ratio and mixed in a ball mill. Next, the obtained powder mixture is placed in a MgO pot and burned in air at 1200 to 1350.degree. C. for 3 to 10 hours. Subsequently, the burned material is crushed using a ball mill to form a BaTi, SrTi, or BaSrTi compound oxide powder. Here, such a compound oxide powder can also be prepared according to hydrothermal synthesis, or can be commercially obtained.
Next, the obtained BaTi, SrTi, or BaSrTi compound oxide powder is subjected to burning and crushing under the same conditions as above further two times or more, and then hot-pressed in a vacuum of 1.times.10.sup.-4 to 5.times.10.sup.-2 Torr under a load of 50 to 200 kg/cm.sup.2 at 900 to 1300.degree. C. for 0.5 to 3 hours. The thus-prepared sintered material of the BaTi, SrTi, or BaSrTi compound oxide is then processed by grinding into a target having a predetermined size.
As shown in FIG. 12, such a process for grinding a sintered material of a BaTi, SrTi, or BaSrTi compound oxide is usually carried out by placing a rotating straight grinding wheel 2' into perpendicular contact with the surface of the sintered material which is horizontally rotated on a grinding table 3, and moving the rotating straight grinding wheel 2' in the direction of the radius of the grinding table. Scratches 4' due to grinding (grinding scratches) are formed in concentric circles on the surface of the thus-obtained conventional target 1' by such grinding.
As shown in FIG. 10, the obtained target is set in a sputtering system 6 by being soldered onto a water-cooled copper plate 5 and subjected to sputtering under the condition that the region of the system for the target is adjusted to a vacuum atmosphere, thus depositing a ferroelectric film.
Recently, for mass production of DRAMs or the like and for saving the costs of producing the same, a larger target is used and sputtered with a greater power under a high-vacuum atmosphere thereby achieving a high deposition rate, and thus depositing a ferroelectric film in a shorter time. Such a target may, however, suffer damage such as peeling of the surface or cracking when subjected to sputtering with a greater power under a high-vacuum atmosphere. If the target is thus damaged, particles are generated on the obtained thin film, seriously affecting the yield of DRAMs. Thus, there remains a need for a sputtering target which is useful for depositing a ferroelectric film and does not suffer from cracking during sputtering. There also remains a need for a method for preparing such a sputtering target and a method for preparing a DRAM using such a target.